With the development of optical communication technology, development of optical components is becoming more and more important. Above all, an optical transmitter or receiver is increased in its transmission speed and response speed, and has a larger communication capacity. In a configuration of a typical transmitter or receiver, the transmitter or receiver includes a light emitting element or a light receiving element fabricated using optical semiconductor, and an output or input optical fiber, and they are optically coupled to each other via a lens. For example, in a case of an optical receiver, light emitted from the input optical fiber is focused on the light receiving element by the lens, and is directly detected (intensity detection).
Turning to a modulation and demodulation processing technique for an optical transmission system, signal transmission using a phase modulation method is in wide practical use. A phase shift keying (PSK) method is a method for transmitting a signal through modulation of the phase of light. The PSK method can achieve much larger transmission capacity than before by way of multi symbol modulation or the like.
In order to receive such a PSK signal, the phase of light needs to be detected. The light receiving element can detect the intensity of signal light, but cannot directly detect the phase of the light. Hence, a means for converting the phase of light into light intensity is needed. For example, there is a method for detecting phase difference by using interference of light. Information on the phase of light can be obtained by causing signal light to interfere with other light (reference light) and detecting the light intensity of interfering light. There are coherent detection and differential detection. In the coherent detection, a light source prepared separately is used as the reference light. In the differential detection, part of signal light is branched off and is used as reference light, and the signal light is caused to interfere with the reference light. As described above, unlike a conventional optical receiver using only an intensity modulation method, a recent optical receiver using the PSK method needs an optical interference circuit which converts phase information into intensity information through interference of light.
Such an optical interference circuit can be achieved using a planar light circuit (PLC). The PLC delivers superior features in terms of mass productivity, low cost, and high reliability, and can be used as various types of light interference circuit. In fact, as an optical interference circuit used in a PSK optical receiver, an optical delay interference circuit, a 90-degree hybrid circuit, and the like are offered and in practical use. Such a PLC is fabricated by a glass deposition technique such as standard photography, etching, and FHD (Flame Hydrolysis Deposition).
In a specific manufacturing process, first, an under-cladding layer made mainly of silica glass or the like and a core layer having a higher refractive index than the under-cladding layer are deposited on a substrate made of Si or the like. Thereafter, various patterns of waveguides are formed on the core layer. Lastly, the waveguides are embedded by an over-cladding layer. A PLC chip having a waveguide-type optical functional circuit is fabricated by such a process. Signal light is encapsulated in the waveguides fabricated by the above process and propagated within the PLC chip.
FIG. 1 shows a conventional method for optically connecting a PLC and an optical receiver. Simple fiber connection as shown in FIG. 1 is employed as a basic method for connection of a PLC and an optical receiver in a PSK optical receiver. Optical coupling is established by connecting a planar light circuit (PLC) 1, which is connected at its input end to an optical fiber 3a, and an optical receiver 2 to each other with optical fibers 3b. The number of optical fibers 3b used for the optical coupling is determined by the number of output light beams outputted from the PLC. Multiple optical fibers are used for the optical coupling in some cases. For this reason, such a configuration of an optical receiver using optical fiber connection may have too large a size. To overcome such a problem in the configuration, the optical receiver can be reduced in size by coupling the output of the PLC and the input of the optical receiver with no optical fiber interposed therebetween and by integrating all into one package. Such a form of an optical receiver in which the PLC and the optical receiver are optically coupled together directly is called an integrated optical module.
To obtain an integrated optical module, how to fix the PLC chip is particularly important. In a case of optically coupling light outputted from the PLC chip and propagated in airspace to a light receiving element by a lens or the like, if the positional relation among the end of light emission from the PLC chip, the lens, and the light receiving element changes, not all of the light can be received by the light receiving element, causing a loss. Such a loss problem is especially noticeable when ambient temperature changes to change the temperature of the package housing the optical receiver, the temperature of each element, and the like, and their positions change due to the influence of thermal expansion. To achieve optical coupling with low loss, it is necessary that the positional relation among the components does not change, at least not relative to each other, even if ambient temperature or the like changes.
In particular, the PLC chip occupies more area in the optical receiver than the light receiving element by about one to two digits, and is therefore more likely to change in shape due to the thermal expansion. Further, a substrate and a deposited thin-film glass which constitute the planar light circuit are largely different in their coefficients of thermal expansion, and therefore temperature change causes large warpage. For this reason, changes in the position and angle of light emitted from the PLC chip relative to the light receiving element are really problematic. These two changes cause the position and angle of light emitted from the planar light circuit to change, leading to displacement in the optical axis. The displacement in the optical axis deteriorates optical coupling of the PLC chip to the light receiving element, and causes a loss. In order to achieve an integrated optical module, it is important to overcome such displacement in the optical axis or to render the displacement harmless.
FIG. 2 shows an internal structure of a conventional integrated optical module. There is known a method for securely fixing the almost entire bottom surface of the PLC chip so that the aforementioned optical-axis displacement may not occur when temperature changes. In the integrated optical module shown in FIG. 2, a PLC chip 13 in which an optical interference circuit is formed as an optical functional circuit, a lens 14, and a light receiving element 15 are fixed to abase substrate 11 with fixing mounts 12a, 12b, 12c as support members, respectively. An optical fiber 16 and the PLC chip 13 are connected to each other via an optical-fiber fixing component 17. In this integrated optical module, light inputted from the optical fiber 16 interferes in the PLC chip 13, and is then coupled to the light receiving element 15 by the lens 14.
The fixing mount 12a and the PLC chip 13 are fixed together by an adhesive 18 or solder. The almost entire bottom surface of the PLC chip 13 is securely fixed to the fixing mount, so that temperature-related expansion or warpage is suppressed. Further, the lens 14 and the light receiving element 15 are also fixed to their fixing mounts, so that the optical axis may not be displaced when temperature changes.
The configuration shown in FIG. 2 can suppress or sufficiently reduce the optical-axis displacement caused by temperature change, but makes noticeable the change in the properties of the PLC chip due to temperature change. As described earlier, the planar light circuit 13 includes a Si substrate 13a and a silica glass layer 13b which are largely different in their coefficients of thermal expansion, and are likely to suffer from great warpage or thermal expansion when temperature changes. In the configuration shown in FIG. 2, thermal expansion and warpage are suppressed because the entire bottom surface of the PLC chip 13 is fixed.
On the other hand, in this case, a large thermal stress is generated between the Si substrate 13a and the silica glass layer 13b. This stress causes change in the refractive index in the silica glass layer 13b through a photo-elastic effect. In the light interference circuit formed in the PLC chip 13, the lengths of waveguides and the refractive indices are precisely adjusted in order to control interference property. The change in the refractive index caused by the stress brings about a change in an equivalent circuit length to change the properties of an interferometer, and consequently, deteriorates the properties of the optical interference circuit.
If, in order to suppress the change in optical properties by suppressing the occurrence of thermal stress, an elastic adhesive, a soft adhesive such as paste, or a fixing paste is used as the adhesive 18 (see, for example, PTL 1), the aforementioned influence on the optical-axis displacement becomes noticeable, and this causes loss.